English

• Large quantities of waste oil are generated due to the frequent use of oil products in the fast-developing petroleum, machinery manufacturing, transportation, and catering industries [1–3]. Waste oil refers to crude oil products that have been contaminated by physical or chemical impurities after use. The emission of waste oil not only causes significant harm to the environment and ecology but also wastes resources. Recycling waste oil is one promising solution [4]. Water is a common impurity in waste oil, and its removal is a crucial step in the recycling process of waste oil [5,6]. Water mainly exists in waste oil in three forms: dissolved water, emulsified water, and free water. Among these, the amount of dissolved water is very low and negligible. Free water easily gathers and stratifies with the oil phase to form an oil/water mixture, which can be removed easily. By contrast, the effective removal of emulsified water from waste oil is much more challenging due to its small size of several micrometers and the presence of surfactants [7,8].

  Various techniques have been developed to separate the water-in-oil emulsion, such as membrane separation [9,10] and adsorption separation, which is based on porous materials [11] and particles [12]. The membrane separation technique is a promising method for emulsion treatment, displaying low energy consumption, high separation efficiency and minimal secondary contamination. Thus, it has attracted considerable research interest. To date, several superhydrophobic membranes [13–15] and Janus membranes [16–18] have been developed to separate the water-in-oil emulsion. These membranes tend to exhibit good flux (thousands of L m⁻² h⁻¹) and separation efficiency (> 99%) when dealing with water-in-oil emulsions that have a low-viscosity oil phase (hexane, octane, toluene, petroleum ether, dodecane, etc. viscosity: 0.3−3.8 mPa/s) [19–22]. However, membrane separation technology remains challenging when it comes to separating water-in-oil emulsions contains highly viscous oil phases, such as crude oil, mineral oil, transformer oil. The elevated viscosity of the oil phase in water-in-oil emulsion causes a dramatic reduction in flux, often by one to two orders of magnitude [19,23]. Additionally, membrane fouling and the resulting flux decline remain challenges that need to be addressed.

  In comparison to the two-dimensional materials used for membrane separation, porous materials (foam [24], sponge [25,26], gel [27], etc.) used for adsorption separation typically display a three-dimensional structure with high a void ratio, low density, large specific surface area, and intricate internal channels. These properties endow them with an excellent adsorption capacity (dozens of times their own weight) [28,29]. Such characteristics allow them to effectively separate oil/water emulsions with a high separation efficiency of >98% [30]. However, several issues remain with porous materials, such as complex preparation processes, high costs, and difficult regeneration, which limit their practical application [11,31].

  In recent years, some researchers have attempted to separate oil/water emulsion using micro/nano-scale particles, such as magnetic iron oxide [32], ZIF-8 particles [33], and functionalized waterworks sludge particles [34]. Typically, superhydrophilic particles are employed to remove water from water-in-oil emulsion. For instance, calcium sulfate hemihydrate crystal [35,36], and MgAl layered double oxide [37] can trap liquid water through a solid-to-solid phase transition in water-in-oil emulsions, and the products can still be recycled as sources for re-preparation. However, their performance remains unsatisfactory due to their relatively lower water capacity (0.63 g/g_{MgAl LDH} [38]). Furthermore, the use of Na₂EDTA as crystallization inducer or the requirement for strict reaction conditions (e.g., hydrothermal method) during the preparation of these particles may cause secondary pollution and increase costs.

  Herein, anhydrous sodium sulfate (Na₂SO₄, ASS) is developed via an evaporative crystallization method for efficient water separation from water-in-oil emulsion. Furthermore, inexpensive commercially available ASS was used as precursors, and no other chemical reagents are required to induce the crystallization process. When mixing ASS with the water-in-oil emulsion, ASS reacts with H₂O and transit into sodium sulfate decahydrate (Na₂SO₄·10 H₂O, SSD) through the reaction:

  $ \mathrm{Na}_2 \mathrm{SO}_4+10 \mathrm{H}_2 \mathrm{O} \rightarrow \mathrm{Na}_2 \mathrm{SO}_4 \cdot 10 \mathrm{H}_2 \mathrm{O} $

  (1)

  Such a phase-transition process immobilizes the water into the crystal lattice of SSD, allowing the water to be separated from the oil along with the removal of SSD. According to Eq. 1, the theoretical water capacity of the ASS lattice can reach 1.27 g/g_ASS, which is more than twice the maximum water capacity reported in the latest literature [38]. More importantly, the reaction product SSD can be used again as a precursor for the preparation of ASS, enabling the recycling of this material. Overall, this work presents an efficient strategy to remove the water from water-in-oil emulsions stabilized by surfactants, offering a promising application in the purification and recycling of many oil products.

  The ASS was prepared by heating the Na⁺ and SO₄²⁻ in a mixed solution of ethylene glycol and water (8.3vol%) under a fixed temperature. The ASS was expected to precipitate out upon the evaporation of water. Figs. 1a–d display typical scanning electron microscopy (SEM) images of the as-prepared ASS at temperatures ranging from 150 ℃ to 180 ℃. The ASS prepared at 150 ℃ (Fig. 1a) presents a granular morphology similar to commercial Na₂SO₄ (Fig. S2 in Supporting information), but with a much smaller size (diameter: approximately 5–15 µm). Both ASS150 and commercial Na₂SO₄ exhibit the same crystal structure as orthorhombic Na₂SO₄ (Form Ⅰ, JCPDS No. 37–1465) according to the X-ray diffraction (XRD) results shown in Fig. 1e. As the temperature increases to 160 ℃, most of the ASS shows a rod morphology, with a minor portion retaining granular shapes (Fig. 1b). The XRD pattern indicates that ASS160 has a mixed phase of orthorhombic Na₂SO₄ (Form Ⅰ, JCPDS No. 37-1465) and another orthorhombic Na₂SO₄ (Form Ⅱ, JCPDS No. 24–1132). This suggests that the morphology of ASS is closely related to its crystal structure, as further evidenced by the SEM images and corresponding XRD patterns (Fig. 1e) of ASS170 and ASS180. In ASS170 and ASS180 samples, almost all crystals exhibit rod morphologies with lengths of approximately 10−30 µm and widths <5 µm (Fig. 1, Fig. 1), and their XRD patterns match well with the structure of Na₂SO₄ (Form Ⅱ, JCPDS No. 24–1132).

  Figure 1

  

  Figure 1.  Typical SEM images (a–d) and corresponding XRD patterns (e) of the as-prepared ASS at different temperatures: (a) 150 ℃, (b) 160 ℃, (c) 170 ℃, (d) 180 ℃. Typical TEM images and corresponding EDS mapping of the as-prepared ASS150 (g) and ASS170 (f).

  DownLoad: Full-Size Img PowerPoint

  The TEM images in Fig. 1, Fig. 1 confirm the granular morphology of ASS150 and the rod-like morphology of ASS170, as well as their different crystal structures. The corresponding EDS mapping reveals that the elements present in the as-prepared ASS are Na, S and O. Moreover, the thermogravimetry (TG) curves (Fig. S3 in Supporting information) of ASS prepared at various temperatures show no weight loss when it is heated from room temperature to 500 ℃, indicating that no water of crystallization is present in the as-prepared ASS. These findings confirm the as-prepared ASS to be the phase of anhydrous sodium sulfate.

  To understand the effect of crystal structure on water separation efficiency, ASS150 and ASS170, which possess different single crystal structures, were selected for further study. The separation experiment was conducted by introducing ASS, in the form of dried powders, into a water-in-oil emulsion (0.1 g H₂O and 0.01 g Span 80 in 10 mL of transformer oil). This mixture was then intensively stirred for 30 min at room temperature to facilitate the immobilization of water within the crystal lattice of ASS. Subsequently, the powders were removed by mild centrifugation (3000 rpm/3 min).

  As shown in Fig. 2a, the water-in-oil emulsion is milky white, and a large number of water droplets, serval micrometers in size, can be observed under an optical microscope. After separation using ASS150 and ASS170, the oil becomes transparent, with almost no water droplets remaining (Fig. 2, Fig. 2). The water separation efficiency of ASS150 and ASS170 reaches 98.41% and 98.63%, respectively. Additionally, both ASS160 and ASS180 exhibit excellent water separation ability (Fig. S4 in Supporting information). SEM characterization of the collected powders after water separation using ASS150 (Fig. 2d) and ASS170 (Fig. 2e) reveals that both ASS150 particles and ASS170 rods have transformed into fragments with no defined morphology, ranging in size from several to dozens of micrometers. The XRD patterns in Fig. 2f show that most peaks of these fragments match well with those of SSD (JCPDS No. 11–0674), with a small number of peaks corresponding to ASS. This indicates that the collected powders are primarily composed of SSD, with a small proportion of ASS remaining, demonstrating effective water immobilization in ASS150 and ASS170. The TG patterns (Fig. 2g) display the typical crystal water removal profile of the collected powders, with weight loss of approximately 44.3% and 44.1%, respectively. These results confirm that ASS150 and ASS170 have transitioned to the SSD phase. Based on the TG data, the separation efficiency (η_s), phase transition efficiency (η_p) and water removal efficiency (η_w) for ASS150 and ASS170 are compared. As shown in Fig. S5 (Supporting information), ASS170 exhibits slightly higher separation efficiency, phase transition efficiency, and water removal efficiency than ASS150. This suggests that ASS170 is more likely to combine with water droplets and transform into SSD during the separation process. This behavior may be attributed to the differences in the crystal structure of the ASS, the calculations demonstrate that the energy barrier for the phase transition from ASS170 (7.85 eV) to SDD is lower than that of ASS150 (8.29 eV).

  Figure 2

  

  Figure 2.  Digital images and corresponding optical microscope images of water-in-oil emulsion before water separation (a) and after water separation by using ASS150 (b) and ASS170 (c). SEM images (d, e), XRD pattern (f) and TG curves (g) of collected powder after water separation by using ASS150 (d) and ASS170 (e).

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  Additionally, the water removal ability of commercial Na₂SO₄ was compared using the same separation method as ASS. The results show that the water-in-oil emulsion remains milky white after separation, and numerous small water droplets can still be observed (Fig. S6 in Supporting information). This indicates that commercial Na₂SO₄ cannot effectively remove water from the water-il-oil emulsion. Fig. 3 presents the optical images of the water-in-oil emulsion during the stirring process after the addition of ASS170. The images shows that water droplets begin to grow with increasing stirring time, and the average size of water droplets reaches its maximum after 12 min of stirring, with the largest water droplet having a diameter of >24 µm (Fig. 3e). Subsequently, the number density and size of water droplets decrease gradually with continued stirring (Figs. 3f–k), and only a small number of tiny water droplets are present after 30 min of stirring (Fig. 3k). After centrifugation (Fig. 3l), almost no water droplets can be observed, indicating the high separation efficiency of ASS170 in removing water droplets from the emulsion.

  Figure 3

  

  Figure 3.  The optical images of water-in-oil emulsion after adding ASS170 and stirring for 0–30 min (a-k) and centrifugation (l). bar = 50 µm.

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  The water removal process from water-in-oil emulsion is illustrated in Fig. 4a. Based on the size variation of water droplets, the entire process can be divided into two stages. At stage Ⅰ (stirring for 0 − 12 min), the merging and growth of water droplets primarily occur. Interestingly, some ASS powders can be found around aggregated or large-sized water droplets (marked with red arrows in Fig. 3). This is due to the driving force between the water droplets and the superhydrophilic ASS170 with high surface energy, causing the droplet to move spontaneously toward the ASS170. This driving force is described by the following equation (Eq. 2) [14,39]:

  $ F \approx \gamma_{\text {water }}\left(\cos \theta_{\text {ASS170 }}-\cos \theta_{\text {oil }}\right) $

  (2)

  Figure 4

  

  Figure 4.  (a) Schematic illustration of the separation process of water droplets from water-in-oil emulsion after adding ASS. (b) Crystal structural transition from ASS to SSD with a view in z-direction.

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  where γ_water represents the surface tension of water droplets, and θ_ASS170 and θ_oil are the contact angles of water droplets and oil droplets in the air on the surface of superhydrophilic ASS170, respectively. As shown in the contact Angle test (Fig. S7 in Supporting information), θ_ASS170 ≈ 0°, and θ_oil ≈ 84°, generating a driving force that propels the water droplets toward the ASS170. When the water droplets come into contact with the ASS170, they firmly adhere to the surface of superhydrophilic ASS170, and then the surfactants attached to the surface of the water droplets are replaced by the superhydrophilic ASS170. Moreover, the flow of the continuous phase and the perturbation of water droplets themselves quickly lead to the collapse and rupture of the oil film on the surface of water droplets. Ultimately, the phase interface between the water droplets disappears, and the small water droplets gradually coalesce into large droplets, promoting the demulsification process [39].

  At stage Ⅱ (stirring for 12−30 min), the water droplets gradually become smaller and eventually disappear, indicating that the phase-transition of ASS to SSD predominates at this stage, leading to the immobilization of water molecules within the crystal lattice of SSD. The immobilized water is then separated from the oil along with the SSD powders after centrifugation. Fig. 4b illustrates the crystal structure transition from ASS to SSD, including the water capture, viewed in the z-direction. In ASS, two chains (−Na⁺−SO₄²⁻−SO₄²⁻−Na⁺−2Na⁺− and −2Na⁺−Na⁺−SO₄²⁻−SO₄²⁻−Na⁺−) are arranged along the x-direction forming its unit cell, with a distance of 2.8 Å between the two chains. In SSD, the internal sequence of the two chains changes to (−SO₄²⁻−SO₄²⁻−Na⁺−Na⁺−Na⁺−Na⁺−) and (−Na⁺−Na⁺−Na⁺−Na⁺−SO₄²⁻−SO₄²⁻−), and the spacing between the two chains becomes increases to 5.5 Å. Water molecules are immobilized between the two chains, forming a sandwich-like structure. The transition in the internal sequence of the (−Na⁺−SO₄²⁻−) chain and the increased spacing between the chains in crystal structure from ASS to SSD provide more space to accommodate water molecules. This is proposed to be the reason why ASS can immobilize water droplets through phase transition.

  The as-prepared ASS in this work exhibits remarkable separation efficiency compared to commercial Na₂SO₄ for the water droplets in emulsion, which can be attributed to the size of Na₂SO₄ particles. The size of commercial Na₂SO₄ is approximately 200−500 µm (Fig. S2), significantly larger than that of water droplets (several microns). This size discrepancy makes it difficult for water droplets to adsorb on the surface of commercial Na₂SO₄ particles, severely inhibiting the aggregation and merging process of water droplets, thereby preventing the demulsification process. Moreover, water immobilization into the crystal lattice is an interface-based reaction [36], the excessive size gap results in a less effective contact area between commercial Na₂SO₄ particles and water droplets, thus hindering the phase transition process of Na₂SO₄. Consequently, numerous tiny water droplets can still be observed in the emulsion after the separation process by using commercial Na₂SO₄ (Fig. S6). In contrast, the size of the as-prepared ASS particles in this work is <30 µm, and these particles exhibit a much larger BET specific surface area (5.2226−6.1990 m²/g) compared to commercial Na₂SO₄ (0.8530 m²/g, Fig. S8 in Supporting information). Thus, water droplets tend to adsorb more easily to the surface of the superhydrophilic ASS particles. As water droplets accumulate on the surface of the ASS particles, they collide and coarsen into larger droplets (Figs. 3a–e), resulting in a larger effective contact area between the droplets and the ASS particles. Subsequently, the ASS particles begin to capture water molecules and then phase transition into SSD, causing these large water droplets to gradually become smaller again (Figs. 3e–k). These results emphasize the superiority of small-sized ASS particles in the demulsification of surfactant-stabilized water-in-oil emulsion by capturing tiny water droplets and combining them into larger droplets, facilitating the phase transition of ASS.

  Considering that there is almost no difference in the water separation efficiency for ASS with various crystal structures, one of them (ASS170) was selected to further investigate other factors that may affect its water separation efficiency. Fig. 5a shows the variation of separation efficiency with the dosage of ASS170. According to Eq. 1, about 8 g of ASS170 is theoretically required to remove the water from 1 L of water-in-oil emulsion with water content of 10 g/L. However, the separation efficiency was only 89.75% when 8 g/L of ASS170 was used. This suggests that the theoretical dosage of ASS is insufficient to completely remove water from the emulsion, likely due to the poor dispersion of ASS in the water-in-oil emulsion, resulting in insufficient contact between ASS and water droplets. Increasing the dosage of ASS170 to 10 g/L significantly improved the separation efficiency to 98.63%. Further increasing the dosage of ASS170 to 12 g/L resulted in a slight enhancement of the separation efficiency from 98.63% to 99.98%. This demonstrates that the separation efficiency can be effectively improved by moderately increasing the dosage of ASS, although the cost also rises. Therefore, a dosage of 10 g/L ASS is suitable for practical use, balancing both economy and separation efficiency.

  Figure 5

  

  Figure 5.  Variation of water separation efficiency with (a) dosage of ASS170, (b) volume ratio of Na₂SO₄ solution to EG and (c) the stirring time. And the digital image (d), separation efficiency (e) and XRD patterns (f) for the regenerated ASS170 during the five cycles of oil-water separation.

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  In this study, a Na₂SO₄ solution and EG were used for the preparation of ASS. The effect of their volume ratio on the water removal ability of as-prepared ASS170 was explored. As shown in Fig. 5b, the separation efficiency of ASS170 prepared with various volume ratios of the Na₂SO₄ solution to EG shows no significant discrepancy, with all values higher than 98%. The XRD patterns (Fig. S9 in Supporting information) demonstrate that the crystal structure of as-prepared ASS170 is not affected by the volume ratio of the Na₂SO₄ solution to the EG. Thus, it can be inferred that the water removal ability of ASS is independent of the volume ratio of Na₂SO₄ solution to the EG during the preparation process. Moreover, the effect of stirring time on the separation efficiency of ASS170 is shown in Fig. 5c. The separation efficiency of ASS170 reaches 97.05% when the stirring time is 15 min, and it improves with the extension of stirring time, reaching higher than 99.30% after 60 min of stirring. This indicates that prolonged stirring time provides conditions for the complete reaction of ASS170 with the water droplets in the emulsion, allowing more water droplets to be immobilized in the lattice of ASS170. Additionally, other factors may also influence the separation efficiency. For example, higher operating temperatures can enhance the separation efficiency of particles [38].

  The XRD patterns in Fig. 2f show that the collected solid powders after oil-water separation consist of Na₂SO₄·10 H₂O and Na₂SO₄, which can be substituted for commercial Na₂SO₄ as the source of Na⁺ and SO₄²⁻ to prepare ASS. The regeneration of ASS is the same as the synthesis method of ASS as described in experimental section. The digital image shows that the water-in-oil emulsions become transparent after separation (Fig. 5d), indicating that the regenerated ASS170 still possesses excellent water removal ability. Fig. 5e shows that the separation efficiency of cyclically regenerated ASS170 is very close to that of initially prepared ASS170, and it remains stable above 96.30% after five cycles. Additionally, the XRD patterns (Fig. 5f) show that the crystal structure of regenerated ASS170 remains unaltered during the cyclic regeneration process. All of these results demonstrates that the excellent renewable properties of the ASS170.

  The economy is a critical factor in achieving the industrial application of oil-water separation technology. Here, the economy of the ASS170 is evaluated by calculating the total cost of the chemical and energy consumptions during the preparation and regeneration process. Table S1 (Supporting information) shows that the preparation of each ton of ASS170 costs ＄9018.31. Considering the effect of the dosage of ASS170 on separation efficiency, Table S2 (Supporting information) lists the cost of purifying water-in-oil emulsion with different dosages of ASS170. It costs only ＄66.51 per cubic meter with a separation efficiency of 98.61% when using 10 g/L of ASS170, and the cost increases to $79.81 per cubic meter when the dosage is 12 g/L. The cost of treating water-in-oil emulsion (water content: 10 g/L) and the separation efficiency of ASS were compared with those of other water removal materials in previous literatures [35–38]. As shown in Fig. 6, the ASS prepared in this work is not only more efficient in water removal but also more economical compared to other materials.

  Figure 6

  

  Figure 6.  Comparison of the cost for treating water-in-oil emulsion and separation efficiency by using ASS in this work with other water removal materials in previous literatures.

  DownLoad: Full-Size Img PowerPoint

  In summary, we demonstrate the robust performance of as-prepared ASS in the separation of water from emulsions and propose a size-dependent demulsification process for ASS particles, elucidating its separation mechanism. Specifically, 0.1 g of ASS enables the purification of 10 mL of emulsion containing 0.1 g of water, achieving a separation efficiency of 98.63%. The ASS can be regenerated from the collected product after oil-water separation, and the regenerated ASS maintains efficient and stable separation performance. This significantly reduces the cost of ASS compared to other water removal materials. The exceptional water-in-oil emulsion separation efficiency of ASS, combined with its facile and environmentally friendly preparation method, positions it as a promising technology in the field of oil purification and recycling.

  Declaration of interest statement

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  CRediT authorship contribution statement

  Yan Zou: Writing – original draft, Funding acquisition, Data curation. Yuting Xue: Investigation. Chenxue Du: Validation, Investigation. Wenyang Fu: Software. Bin Xia: Methodology. Yu He: Funding acquisition. Liang Ao: Resources. Xiaoshu Lv: Formal analysis. Guangming Jiang: Writing – review & editing, Conceptualization.

  Acknowledgments

  The present work is financially supported by the High-level talent research start-up project of Chongqing Technology and Business University (No. 2356007), the Science and Technology Research Program of Chongqing Municipal Education Commission (No. KJQN202400809) and Special Project for Performance Incentive and Guidance of Research Institutions in Chongqing (No. CSTB2023JXJL-YFX0030).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2025.110814.

  
  
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• 

  Figure 1  Typical SEM images (a–d) and corresponding XRD patterns (e) of the as-prepared ASS at different temperatures: (a) 150 ℃, (b) 160 ℃, (c) 170 ℃, (d) 180 ℃. Typical TEM images and corresponding EDS mapping of the as-prepared ASS150 (g) and ASS170 (f).

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  Figure 2  Digital images and corresponding optical microscope images of water-in-oil emulsion before water separation (a) and after water separation by using ASS150 (b) and ASS170 (c). SEM images (d, e), XRD pattern (f) and TG curves (g) of collected powder after water separation by using ASS150 (d) and ASS170 (e).

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  Figure 3  The optical images of water-in-oil emulsion after adding ASS170 and stirring for 0–30 min (a-k) and centrifugation (l). bar = 50 µm.

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  Figure 4  (a) Schematic illustration of the separation process of water droplets from water-in-oil emulsion after adding ASS. (b) Crystal structural transition from ASS to SSD with a view in z-direction.

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  Figure 5  Variation of water separation efficiency with (a) dosage of ASS170, (b) volume ratio of Na₂SO₄ solution to EG and (c) the stirring time. And the digital image (d), separation efficiency (e) and XRD patterns (f) for the regenerated ASS170 during the five cycles of oil-water separation.

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  Figure 6  Comparison of the cost for treating water-in-oil emulsion and separation efficiency by using ASS in this work with other water removal materials in previous literatures.

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